A list of referenced documents is appended to the end of the Description.
Piezoelectric resonators are electro-mechanical resonators. The resonance is due to shape, density and elastic constants of the mechanically resonating body containing piezoelectric material. The piezoelectric effect permits to couple the mechanical resonance to an electric circuit. The use of piezoelectric thin films allows very large resonance frequencies in the 1-10 GHz range by trapping a bulk acoustic wave within the piezoelectric film slab. This is of great interest for wireless communication. Thin film piezoelectric resonators are indeed commercialized for applications in mobile phones. Main product is the RF filter at the carrier frequency in the receiver and emitter path. Such filters need a large pass band, meaning large piezoelectric coupling. For this reason, the largest piezoelectric coefficient is employed, i.e. the electric field is applied parallel to the polar c-axis (coefficient e33). It happens that AlN and ZnO films can be very well grown in (001) texture, that is with the c-axis (index 3 in the crystal system) perpendicular to the film plane (this direction is designed with index z of the coordinate system). The electric field is then very conveniently applied between two parallel plate electrodes sandwiching the piezoelectric film. The piezoelectric effect is then written for instance as change of stress Tz{grave over ( )}−e33Ez. Using this mode, a longitudinal wave running along the 3-direction is trapped in the film slab, excited through the largest possible coupling based on e33. Informations on this kind of devices are found for instance in the articles: Lakin [1], Ruby [2], Lanz [3].
Another promising application of the thin film bulk acoustic wave resonators (TFBAR's) has been identified in gravimetric sensing. High quality factor and high frequency make such device very sensitive to any particles or films that agglomerate at the surface of the device. However, such sensors are not able to operate in a liquid when using longitudinal waves as for mobile phones. The liquid is damping too much the resonance because longitudinal waves are emitted from the resonator into the liquid. Contrary to that, resonators that operate on shear acoustic waves will not be damped so much, because there is only a weak shear coupling into liquids. Shear waves do not propagate in liquids, and thus do not absorb energy from the resonator. A schematic view of a shear mode resonator combined with an immobilization layer to obtain gravimetric sensing is shown in FIG. 1.
Excitation of shear waves in thin films was proposed in several ways. A first solution is tilted c-axis growth (Wingqvist, (Sensors, 2005 IEEE)[4]). The electrode geometry is the same as for mobile phone RF filters, however, the c-axis in the piezoelectric film is tilted away from the vertical (direction 3) by the angle alpha. In this geometry, quasi-shear waves running along the 3-direction are excited by an electric field pointing along the 3-axis, and are trapped in the film at resonance. The main disadvantage of this technique is the need for non-standard deposition tools and large difficulties to achieve uniform c-axis tilting. A second solution is to provide an in-plane electric field by means of interdigitated electrodes along with standard (001) AlN films. The advantage is thus that standard films with good uniformities can be used. The disadvantage is the creation of an S3 component of strain below the electrode fingers, leading to a longitudinal wave component, and thus to emission into the liquid. The quality factor is thus not as optimal as it could be, even though better Q's were observed than with tilted c-axis growth (see article [6] of the inventors).
The publication Martin [5] discloses background information on growing thin AlN films, in particular with differently treated areas yielding different polarization.